1. Field of the Invention
This invention relates to photopolymerizable elements and methods of photoimaging and more particularly to such elements and methods useful in preparing lithographic plates.
2. Relation to the Prior Art
Photopolymerizable compositions and elements useful to form images are well known in the art. Many references in the literature and patent art have described photopolymerizable elements and the initiator systems, monomers, the photopolymerization reactions, the role of oxygen and other variables and components that make up such elements. The elements have many described uses with a particularly preferred use being in lithographic printing.
While photopolymerizable lithographic plates are available commercially, these plates usually require cover sheets or topcoats to prevent oxygen inhibition. Also, it is always desirable to improve image fidelity as indicated by halftone dot quality and to improve photospeed, especially in air.
British Pat. No. 1,006,587 describes ethylenically unsaturated polyhydroxy polyethers as compositions, and the U.S. equivalent (U.S. Pat. No. 3,373,075) claims a fibrous impregnating composition comprising a solution of the ethylenically unsaturated compound and a thermal, addition-polymerization initiator.
Photopolymerizable compositions which contain ethylenically unsaturated diester polyhydroxy polyethers are known. Crary (U.S. Pat. No. 3,661,576) describes such compositions and photoresist articles which use an addition polymerization initiator. German Pat. No. 2,541,133 describes such compositions which also contain an N-nitrosodiarylamine to minimize thermal polymerization. Japanese published Patent Application No. 77,007,362, published Mar. 2, 1977, also describes such compositions useful for printing plates which also contain alkali soluble copolymer resin capable of being addition polymerized with the diester through a photochemical reaction.
Collier et al., (U.S. Pat. No. 3,984,244) describes a channeled photosensitive element, useful as a solder mask for printed circuit boards. Among the suitable monomers described is polyoxyethyl-2,2-di(p-hydroxyphenyl)propane dimethacrylate. Among the suitable addition polymerization initiators disclosed is 2,4,5-triarylimidaozolyl dimers with hydrogen donors.
Assignee's copending application (Gervay et al., U.S. Ser. No. 735,979, filed Oct. 27, 1976) discloses a halogen-containing photopolymerizable composition which comprises an ethylenically unsaturated monomer including ethylenically unsaturated diester polyhydroxy polyether, an initiator system including 2,4,5-triarylimidazolyl dimer plus hydrogen donor polymerization initiator, and a selected binder which contains polyacrylonitrile. The compositions are useful in solder mask and other photoresist applications.
Chang et al. (U.S. Pat. No. 3,549,367) describe an addition-polymerization photoinitiating system based on a mixture of 2,4,5-triarylimidazolyl dimer and a p-aminophenyl ketone.
None of the art discloses a photopolymerizable element, useful in lithographic printing, comprising a substrate bearing a photopolymerizable layer containing an ethylenically unsaturated diester polyhydroxy polyether and an addition-polymerization photoinitiator system based on a mixture of 2,4,5-triarylimidazolyl dimer and a p-aminophenylketone.